Stretch blow moulding (SBM) is a process of producing hollow thermoplastic articles, for example containers. This process involves the initial production of a preform article by injection blow moulding. The preform article is then subjected to biaxial orientation by axially stretching the preform using an external gripper or an internal stretch rod, and then by radially stretching it by blowing compressed air into the preform and pushing the material against a mould to form a hollow thermoplastic article.
The biaxial orientation provides the stretch blow moulded article with improved physical properties. The process aligns the molecules along two planes providing additional strength and better barrier properties than is possible using other blow moulding techniques. Other advantages include better clarity, increased impact strength, and reduced creep.
Stretch blow moulded articles can be obtained using two different stretch blow moulding processes. During the so-called “single-stage process” or “in-line process”, the preform and blow moulded articles are prepared on the same machine. During the “two-stage process” or “reheat process” the preforms are formed during the first stage on an injection machine and then left to cool to ambient temperature. After this stage, the preforms may be transported from one location to another. In the second stage, the preforms are reheated and stretch blow moulded into the desired article on a reheat-blow machine. The present invention can be applied to both single-stage and two-stage stretch blow moulded containers.
There has been a long felt need in the industry of SBM manufacturing to obtain polypropylene containers, which have a high drop impact resistance. Consequently, there is also a need to develop a method, which would allow one to rapidly and easily determine the drop impact resistance of a polypropylene container.
The drop impact resistance can be determined using tests such as the standardised test D2463-95 devised by the American Society for Testing and Materials (ASTM) or the test devised by the International Society of Beverage Technologists (ISBT). However, these are long processes, which requires many containers, filling them up, conditioning them and then dropping them on a plane in several different inclinations using special drop impact resistance equipment.
US 2005/0249905 discloses a free shrinkage test, which is a method of predicting and estimating the processing parameters of a polypropylene container made by a two-stage process. This test involves measuring the wall thickness and shrinkage of a container after immersing it in a hot oil bath. It discloses carrying out free shrinkage tests for containers obtained using the two-stage stretch blow moulding process and containers consisting essentially of polypropylene random copolymer. However, it does not describe how the drop impact resistance can be obtained. In addition, carrying out repetitive tests in hot oil is neither safe, nor user-friendly.
There is thus a need to provide a rapid and easy method to predict the drop impact resistance of a polypropylene container obtained from either the one-stage or the two-stage stretch-blow moulding process.
It is therefore an aim of the present invention to provide a method to predict the drop impact resistance of a polypropylene container.
It is another aim of the present invention to provide a rapid method to predict the drop impact resistance of a polypropylene container.
It is yet another aim of the present invention to provide an easy method to predict the drop impact resistance of a polypropylene container.
Furthermore, it is an aim of the present invention to provide a method to predict the drop impact resistance of a polypropylene container obtained from a one-stage stretch-blow moulding process.
It is also an aim of the present invention to provide a method to predict the drop impact resistance of a polypropylene container obtained from a two-stage stretch-blow moulding process.
One or more of these aims has been, at least partially, fulfilled by the present invention.